Originally Posted by
mcours2006
Radar is also useful, but when I commute I don't bother using my bike computer, or any other kind of head unit.
I don't own a bike computer, but I do have a Garmin Varia. The Varia has a free phone app. that works well and is simple. The nice thing about using the app - with a mirror - is that as long as you can hear your phone you know what to expect and when to look at the mirror - don't need to see the phone - more of your attention can be focused forward. The app. makes 3 different sounds that indicate car approaching, car approaching at high speed, and all clear.
After hearing them a few times, you quickly learn what they mean. You can keep your phone in your pocket, handlebar bag, etc. - don't need to mount it on your bars.